May. 06, 2024
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Semiconductors undergo many processes to become functional for the various applications they are utilized in. Sputtering is a process that is used in many industries, but what exactly is it and why is it used?
In particular, semiconductor sputtering is a process that is used to deposit thin films of material onto a substrate. There are several reasons why this process might be used in the semiconductor industry, including creating device contacts, forming resistors and capacitors, and manufacturing transistors. Let’s take a closer look at semiconductor sputtering and find out why it’s such an important part of the semiconductor fabrication process.
Sputtering is the process of ejecting surface atoms of a solid target material that shift from their solid phase to their gas phase after it has been bombarded with energetic ions. The kinetic energy of the incident ions must be higher than the target surface binding energy for sputtering to take place. This is a physical vapor deposition method that renders a thin film and coating on the sputtering target material or ‘substrate’, most often inside a vacuum chamber. The average quantity of target atoms that leave the surface for each incident ion is known as the sputter yield.
In layman’s terms, this process involves a material being bombarded by particles containing energy in the form of plasma or inert gas. This moment of collision results in particles being ejected from the surface of the solid material that was bombarded. The conventional thermal energies should be less than that of the ion bombardment that acts upon it. This procedure usually takes place in outer space on its own accord but we humans have used it in various applications down here too.
There are many different types of sputtering, from DC sputtering using a direct current to magnetron sputtering, reactive sputtering, RF sputtering, and more. The ability to carry out sputtering on thin film materials has helped in the fabrication of many devices and equipment ranging from nanotechnology materials to optical material coatings, semiconductor devices, and more.
We’ll look into the details of sputtering in the semiconductor scope right below, from how the process is carried out to its importance and drawbacks
In the semiconductor industry, sputtering target materials are required for many procedures of the semiconductor manufacturing process. This includes the wafer fabrication process dealing with the wafer conductive layer, barrier layer, and more. The sputtering process is also used in the semiconductor chip packing process.
The main methods of sputtering used are ion beam sputtering for sputter etching in semiconductors and reactive sputtering. But, why is sputtering so important for semiconductors?
Technological advances and growing consumer demands for more lightweight devices requiring less power and better processing means that the sputtering techniques and targets used in the fabrication of semiconductors must help improve these factors.
Sputtering is used in both wafer fabrication and chip packaging as mentioned above. However, semiconductors are very sensitive devices and require high purity for both water and chip production due to how easily particles form on their surfaces. To meet consumer demands, this means that the quality of the sputtering target material must be at its best, which is usually 99.9995% (5N5) or higher.
The effectiveness of the sputtering process directly correlates to the quality of metal target materials. High quality affects many aspects of semiconductor fabrication, as the purity of the sputtering target materials will directly show in the uniformity and conductive capabilities of the conduction layer and barrier layer. Consequently, this also affects the momentum transfer and stability of chip transmission.
There are many different types of sputtering target materials and evaporation materials used in semiconductor fabrication, which we will discuss in detail below.
Various elements can be used as sputtering target materials to be bombarded with different forms of high-energy particles. These elements have different levels of purity and different forms like monoblocks and compounds of varying sizes. They also differ in the cathode materials they utilize for the process and undergo different mounting methods. Each sputtering target differs in its use case and is only suitable for a specific sputtering target system with particular coating materials.
Below are most of these elements, but note that they are not functional across all applications.
Sputtering has many advantages but not without a few drawbacks. This physical process needs the best sputtering equipment and high-purity target materials to ensure the silicon wafer or other wafer material does not get damaged quickly.
If the sputtering process does not happen under controlled conditions and with high-purity sputtering targets, it will easily cause a film on the wafer. This will lead to short circuits or irrecoverable circuit damage.
This process requires high ion energy and it can be quite an exhausting process to arrange and carry out.
Sputtering and etching are key processes for the fabrication of semiconductor devices. In this article, we’ve looked at what sputtering is and how it works in the semiconductor industry. We’ve also seen that sputter deposition is an important step in fabricating high-quality semiconductors and that sputter etching makes these devices more efficient. While there are some drawbacks to using sputtering technology – wafer damage and high voltage use – the benefits of faster, better quality products outweigh these concerns.
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Sputtering is widely used in the field of semiconductor manufacturing, to form thin layers of different materials as part of integrated circuit processing. Additionally, this technology can be leveraged to deposit optical antireflection coatings on glass for numerous applications.
RF magnetron sputtering is a superior method compared to DC magnetron sputtering because it doesn’t require an electrically conductive target. This allows any material, in theory, to be sputter-deposited using this revolutionary technique.
Sputter deposition involves ejecting material from a target to a substrate through plasma generated by inert gas, forming uniform thin films.
Sputtering is crucial for IC fabrication, enabling the deposition of contact metals, barrier layers, and conductive components, ensuring chip performance and stability.
Sputtering target material quality directly impacts semiconductor uniformity, performance, and cost-effectiveness, which are vital for nanoscale components in evolving technology.
Sputtering in ic fabrication diagram.
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Sputter deposition is a type of physical vapor deposition (PVD) method of thin film deposition, used many times as part of the IC fabrication process and in ceramic PCB manufacturing. Sputtering involves ejecting material from a "target" that is a source onto a "substrate," creating a thin film. Sputtered thin films have excellent uniformity, density, and adhesion. Read on as we discuss sputtering in IC fabrication and how it is accomplished.
Application
Purpose
Primary Sputtering Target Materials
Thin-Film Transistors
Deposition of contact metals
High-purity semiconductor-grade sputtered targets
Barrier Layer
Deterrent and insulator. Adhesion point for bonding metal layers
Tantalum and titanium sputtering targets
Wafer Manufacturing
Conductive layer, barrier layer, metal grid
Aluminum, titanium, copper, and tantalum sputtering targets
Conductive Layers
Control of electric current
Aluminum, titanium, copper, and tantalum sputtering targets
Chip Packaging
Metal layer under the bump, wiring layer, and other metal materials
Copper, aluminum, and titanium sputtering targets
Placed within a vacuum chamber, the target material (acting as the source) and the substrate (the intended destination) are subjected to a voltage differential. This voltage setup designates the target as the cathode while the substrate is linked to the anode, as shown in the figure above.
Through ionization of a sputtering gas—commonly an inert gas like argon or xenon—a plasma is generated. Inert gases are favored as sputtering gases due to their minimal reactivity with the target material and process gases and their capacity to yield heightened sputtering and deposition rates owing to their substantial molecular weight.
The sputtering process commences as the sputtering gas bombards the target material. This collision results in an energy transfer that propels target particles, leading them to escape, traverse, and eventually settle onto the substrate as a film. As the gas enters the chamber for sputtering, the following steps occur:
Material is eroded from the target under the bombardment (cathode) of the gas. This phenomenon corresponds to the sputtering mechanism elucidated in this context.
Ions become integrated into the target material, possibly even forming a chemical compound. This phenomenon takes on the name of (reactive) ion implantation.
The ions amass on the impacted substrate, culminating in the creation of a layer. This specific scenario is known as ion beam deposition.
As atoms are expelled from the target, they exhibit a broad energy spectrum, often reaching up to tens of electronvolts (equivalent to 100,000 Kelvin). Among these expelled particles, sputtered ions—typically a minor portion, around 1 percent—are ionized. These ions can travel in direct trajectories from the target, impacting the substrates or the vacuum chamber with substantial energy, thereby inducing resputtering. Alternatively, in scenarios of heightened gas pressures, the ions interact with gas atoms, which act as moderators. This interaction causes the ions to move diffusively, leading them to eventually reach the substrates or the walls of the vacuum chamber. They undergo condensation after undergoing a random walk through this diffusive process.
The range of behavior spans from high-energy ballistic collisions to low-energy thermalized motion, which can be manipulated by adjusting the background gas pressure. Typically, an inert gas like argon serves as the sputtering gas. For effective momentum transfer, the atomic weight of the sputtering gas should closely match that of the target material. Consequently, for sputtering lighter elements, neon proves advantageous, whereas heavier elements like krypton or xenon find application. Reactive gases also find utility for sputtering compounds. Depending on the process parameters, compounds can form on the target surface, during flight, or on the substrate. The multitude of parameters governing sputter deposition contributes to its complexity. However, this complexity grants experts significant control over film growth and microstructure.
Sputtering is used extensively in the semiconductor industry to deposit thin films of various materials in integrated circuit processing. Because of the low substrate temperatures used, sputtering is an ideal method to deposit contact metals for thin-film transistors. Semiconductor chips have high technical requirements and high prices for sputtered targets. Their requirements for sputtered targets' purity and technology are higher than flat-panel displays, solar cells, and other applications.
Although the amount of target materials used in wafer manufacturing and chip packaging is small, the cost of target materials in wafer manufacturing and packaging process accounts for about 3%. However, the quality of sputtering target materials directly affects the uniformity and performance of the conductive layer and barrier layer and further affects chip transmission speed and stability, so sputtering target materials are the core raw materials for semiconductor production one of the materials.
The dimensions of modern transistors and other components have reached nanoscale levels, demanding precise layering of materials to ensure functionality and reliability. Sputtering, with its capability to deposit materials with sub-nanometer precision, is vital in enabling the creation of these highly miniaturized components.
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